Separating particulate solids from a liquid has a multiplicity of applications. They include dewatering of particulate slurries and separation of liquid from solids in the leaching process. The leaching of constituents from particulate mineral materials is practiced utilizing a wide variety of materials and equipment. Leaching procedures are particularly useful for the recovery of metals from particulate mineral ores, such as gold, silver, copper and uranium ores. The dominant process for the extraction of such metals from ores is leaching with lixiviants. Typical leaching methods have a number of drawbacks, in particular the need to either grind the ore finely for continuous agitation leaching or utilize batch leaching methods on coarser material.
Tank leaching is typically a continuous process, while vat leaching is operated in a batch fashion. Tank leaching is commonly used to extract gold and silver from ore. Tank leaching differs from vat leaching in that in tank leaching the material is ground sufficiently finely to form a slurry which can flow under gravity or when pumped, whereas in vat leaching typically a coarser material is placed in the vat for leaching. The tanks in the tank leaching method are typically equipped with agitators to keep the solids in suspension in the tanks and improve the solid to liquid to gas contact. Baffles can be provided to increase the efficiency of agitation and prevent centrifuging of slurries in circular tanks. The vats in vat leaching usually do not contain such equipment. In a tank leach the slurry is agitated, while in a vat leach the solids remain stationary in the vat, and solution is moved, so typically the retention time required for vat leaching is more than that for tank leaching to achieve the same percentage of recovery of the valuable material being leached.
Tank and vat leaching both involve placing the ore, after size reduction and classification, into the tanks or vats at ambient operating conditions containing a leaching solution and allowing the valuable material to leach from the ore into solution. In tank leaching the ground, classified solids are already mixed with water to form the slurry, and this is pumped into the tanks. Lixiviants are added to the tanks to achieve the leaching reaction. In a continuous system the slurry will then either overflow from one tank to the next, or be pumped to the next tank. Ultimately the pregnant solution is separated from the solids using some form of liquid/solid separation process, and the solution passes on to the next phase of recovery. In vat leaching the solids are loaded into the vat and, once the vat is full it is flooded with a leaching solution. The solution drains from the tank, and is either recycled back into the vat or is pumped to the next step of the recovery process.
The factors which affect extraction efficiency are: i) Retention time—the time spent in the leaching system by the solids. This is calculated as the total volumetric capacity of the leach tank(s) divided by the volumetric throughput of the solid/liquid slurry. ii) Particle size—The ore is ground to a size that exposes the desired mineral to the leaching agent. In tank leaching this must be a size that can be fully mixed and suspended by the agitator. In vat leaching this is the size that is the most economically viable, balancing recovery against the increased cost of processing the material. iii) Slurry density—The slurry density (percent solids) determines retention time. The settling rate and viscosity of the slurry are functions of the slurry density. The viscosity, in turn, controls the gas mass transfer and the leaching rate. iv) Dissolved gas—Gas, typically oxygen, may be injected into the solution to obtain the desired dissolved gas levels. vi) Reagents are added and the appropriate amount of reagents maintained throughout the leach circuit to maximize the metal recovery. v) Temperature—impacts the reaction kinetics. vi) Leach-inhibiting elements such as lixiviant-consuming minerals or carbonaceous materials.
The conventional knowledge says that the maximum particle size for agitation leaching should be much less than 1 mm in diameter to permit maximum recovery in a reasonable retention time, as well as allowing fully homogeneous mixing. In gold leaching by carbon capture, using finely ground particles allows carbon separation. Such a fine particle size requires expensive grinding.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.